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Related Concept Videos

Spin–Spin Coupling Constant: Overview01:08

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
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People have observed the rolling motion without slipping ever since the invention of the wheel. For example, one can look at the interaction between a car's tires and the surface of the road. If the driver presses the accelerator to the floor so that the tires spin without the car moving forward, there must be kinetic friction between the wheels and the road's surface. If the driver slowly presses the accelerator, causing the car to move forward, the tires roll without slipping. It is...
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Spin–Spin Coupling: One-Bond Coupling01:17

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Atomic Nuclei: Nuclear Spin01:08

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All atomic particles possess an intrinsic angular momentum, or 'spin'. Electrons, protons, and neutrons each have a spin value of ½, although protons and neutrons in nuclei may have higher half-integer spins owing to energetic factors.
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
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Spin contamination in MP2 and CC2, a surprising issue.

Marios-Petros Kitsaras1, Stella Stopkowicz1

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|April 9, 2021
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Artificial spin contamination in Møller-Plesset (MP2) and coupled-cluster (CC2) calculations is addressed. A new method removes this contamination in closed-shell molecules and improves open-shell results, with CC2 outperforming MP2.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Spin contamination is an artifact in electronic structure calculations, particularly affecting MP2 and CC2 methods.
  • Existing methods for calculating spin multiplicity in MP2 and CC2 theories can lead to artificial spin contamination in closed-shell molecules.
  • This contamination is counterintuitively observed to be stronger in open-shell systems at correlated levels than at the Hartree-Fock reference.

Purpose of the Study:

  • To investigate the underlying reasons for artificial spin contamination in MP2 and CC2 calculations.
  • To propose a novel solution to eliminate spin contamination in closed-shell systems and yield physically meaningful results for open-shell systems.
  • To compare the performance of CC2 and MP2 methods in describing systems with significant spin contamination.

Main Methods:

  • Analysis of the strategy for calculating expectation values and density matrices in MP2 and CC2 theories.
  • Development and application of a new procedure to address spin contamination.
  • Comparative performance evaluation of MP2, CC2, and coupled-cluster with singles and doubles (CCSD) methods.

Main Results:

  • Identified the cause of artificial spin contamination in MP2 and CC2 calculations for closed-shell molecules.
  • Developed a solution that effectively removes spin contamination for closed-shell systems and provides accurate results for open-shell systems.
  • Demonstrated that CC2 significantly outperforms MP2 for strongly spin-contaminated references, achieving performance comparable to CCSD.

Conclusions:

  • The proposed method successfully resolves spin contamination issues in MP2 and CC2 calculations.
  • The findings provide a more reliable approach for calculating spin properties in quantum chemistry.
  • CC2 is shown to be a more robust method than MP2 for systems with significant spin contamination.